Incorporate CV calibration

[grrrwaaa]

See electro-smith/libDaisy#247

[grrrwaaa]

Thinking aloud:

• Hold down encoder (Patch) or SW1 (Field) or Tap (Versio) etc. on boot to enter calibration mode.

• Existing calibration loaded into memory.

• Per jbeda's notes at Calibrating CVs and storing on QSPI libDaisy#247 -- ensure some audio synthesis is running. Could simply be a triangle wave at the most recently-detected voltage.

• (Patch/Versio etc.: turn all knobs fully CCW).

• Show some LED lightshow to demonstrate we are calibrating.

• Go through Each CV input in turn, patching a cable in from a midi controller's CV out.

• (Patch/Field could also hook up the midi out to the midi controller to automate note playing... though there's some variance between manufactures about what MIDI note == 1v, we could just do about 6 octaves to cover most devices).

• Play 1v, (2v, 3v), 4v. Device should be able to detect nearest integer v and store the sensed voltage accordingly. So, play C notes at different octaves. While a voltage is detected, data stored to calibration struct, and oscillator plays current pitch. So, polling, e.g. every 50ms or something.

• Maybe want to add some smoothing to the CV inputs to get a nice average over microfluctuations.

• Move to next CV input.

• Next CV outputs. Tap the encoder/sw1/tap again to toggle to CV OUT calibration

• Different lightshow for outputs.

• Route a CV out N into CV in N (which is now assumed to be calibrated). Device is playing a step sequence of voltages (1v, 2v, 3v, 4v). Detected voltages (if above 0.5v) will trigger updating the stored value for the CV out.

• A tiny bit of delay maybe needed compared to the CV out (a few ms to cover smoothing)?

• Tap again to exit calibration. Calibration stored to QSPI. Now can test playing other notes to listen how well it is tracking.